This is what works for me!
First, sorry if it's stating the obvious, the photo has to be public, it's no use sharing a link for a private photo!
Find the photo you want to share
Click on the arrow in the bottom right hand corner
Choose BBCode from the options
Choose the image size
Copy the URL in that box (Not the one in the address bar)
Post it here, as it is, within the text box
D3 - View from the cable car station - PH by
Paul, on Flickr
If you try to quote this post, you'll see what the photo looks like in text.
I thought I'd written this out before, though it was a couple of years ago
http://thorncyclesforum.co.uk/index.php?topic=4313.msg100608#msg100608That post also rightly points out that you are directing people to your flickr account where they will be able to browse your public photos.